Polymeric cellular solids (foams) nearly eliminate convective heat transfer and thus combine low density with low thermal conductivity, both properties desirable for thermal insulation. Further reduction in the rate of heat transfer is realized with pore sizes below the mean free path of the pore-filling gas (68 nm for air at standard temperature-pressure (STP)). Such mesoporous (2-50 nm) materials include aerogels which typically exhibit poor mechanical properties. Systematic efforts to improve the mechanical properties of polymeric aerogels by crosslinking fibrous cellulose wet-gels with isocyanates have resulted in some improvement of mechanical properties, but have proven time-consuming and inefficient.
Therefore, what is needed are polymeric aerogels having sufficient strength and structural integrity without the need for post-gelation treatment and efficient methods for their production from readily available starting materials. This present disclosure addresses these needs.
Aerogels are porous materials and with small inter-connected pores. The three major types of aerogels are inorganic, organic, and carbon aerogels. Inorganic aerogels can be obtained by supercritical drying of highly cross-linked and transparent hydrogels synthesized by polycondensation of metal alkoxides. Silica aerogels are the most well known inorganic aerogels. Organic aerogels can be synthesized by supercritical drying of the gels obtained by the sol-gel polycondensation reaction of monomers such as, for example, resorcinol with formaldehyde, in aqueous solutions. Carbon aerogels can be obtained by pyrolizing the organic aerogels at elevated temperatures. It is believed that carbon aerogels are the only electron conductive aerogels; therefore since their discovery, they have been suggested for use as electrodes for fuel cells or as super-capacitors because of their mesoporous structure.
Transition metals are induced in carbon aerogels with the goal of modifying structure, conductivity or catalytic activity, due to homogenous distribution of metal nano particles in the 3-D network of carbon. Thus, carbon supported metal aerogels may be potentially invaluable, as they have unique properties of metals as well as carbon aerogels. The metal in metal aerogel is of nano-size as a result of heat treatment; hence, as a consequence, the nanomaterials often exhibit properties atypical of their bulk metal. This may make them promising materials for applications in the preparation of electrodes, batteries, super capacitors, adsorbents, molecular sieves and catalysts, due to their easy preparation, good textural and chemical properties, in addition to their unique properties like high surface area, porosity and low density.
Conventionally, it has been reported that metal-doped carbon aerogels may be prepared by three main strategies. The first is the addition of the soluble metal precursor (metal salts) in the initial mixture. The second involves the use of a resorcinol derivative containing an ion exchange moiety that can be polymerized using sol-gel technique. The repeating unit of the organic polymer contains a binding site for metal ions to ensure a uniform dispersion of dopant metal. The third approach is to deposit the metal precursor on the organic or carbon aerogel by one of the various methods such as incipient wetness, wet impregnation, adsorption, sublimation and supercritical deposition. The drawback of first method has to do with the nature of salt, which effects sol-gel chemistry by changing the pH of initial solution, hence making it difficult to control pore texture of carbon matrix. A few publications have suggested that doped metal particles are anchored to the carbon structure of monoliths, where micropores act as nucleation sites for the metal nano particles. The anchoring of metal particles block micropores, hence the surface area of carbon aerogel decreases. The polymerization has been reported to be of two kinds: addition and condensation; the former is catalyzed by base and latter with acid, as the pH influences the molecular environment of the initial mixture polymerization, so it plays a major role in determining the structure and pore texture of resultant gel. The anions of salts used as either polymerization catalyst or ion exchange process, and even as metal precursors, also have been reported to have some effects on the sol-gel chemistry as well as on the resulting gel.
It has also been reported that transition metals and organometallic compounds are good catalysts to induce graphitization of carbon infrastructures during carbothermal process. The solid state pyrolysis is believed to be a convenient way to produce graphitic nano structures. It has been reported that the structure and morphology of the derived nano structured carbons may be tuned by changing the type of carbon source, metal catalyst and conditions of carbonization.
Ferrocene is an organometallic compound with two cyclopentadienyl moieties sandwiching Fe (II). There has been interest toward the incorporation of ferrocene into polymer backbone, due to the high stability, redox properties, electrical, conducting/semiconducting, magnetic, optical, catalytic, and elastomeric properties that can be used for a broad range of applications, such as, for example, electronic devices, formation of redox gels with charge transfer properties, modification of electrodes, and in medical applications for cancer treatment. This interest in ferrocene may be attributable in part to the rich chemistry of iron (II) centers and broad range of synthetic methods available for functionalization of the cyclopentadienyl ligands.
Conventionally, aromatic diamines have been known to be valuable building blocks for the preparation of polyamides that are used to produce desired alterations in the chemical nature of a macro chain. It has been reported that the properties of polyamides may be modified further by the addition of metal in their core structure. Inclusion of metals in the polymers may create the possibility of producing specialty materials with useful electrical, magnetic or catalytic properties, combined with thermal stability. Thus, inclusion of the ferrocene entity in the polyamide core structure along with flexible linkages may create the possibility of accessing materials that are superior to conventional polyamides with respect to their better balance of physicochemical properties.
Literature reports have described an ordered mesoporous carbon (i.e., well-ordered porous structure with uniform sized mesopores along with narrow pore size distribution in regular carbon frameworks) containing ferrocene derivative using furfuryl alcohol as the main carbon source; but its surface area was very low. There have also been other reports describing a route to make iron and iron oxide filled carbon nano tubes using ferrocene as precursor. The immobilization of ferrocene in layered or porous materials, motivated by the potential application of these materials in fields such as catalysis, sensors, optical devices, etc., has led to materials in which the chemical and physical properties of both the matrix and organometallic entities were often modified. The pyrolysis of ferrocene in argon atmosphere has been reported to produce a very large amount of carbon nano tubes. Another literature report has described the synthesis of ordered mesoporous carbon containing iron oxides by using ferrocene carboxylic acid as metal precursor and sucrose as main carbon precursor. Ordered mesoporous carbon (OMC) was compared with iron modified ordered mesoporous carbon (Fe-OMC) for their electroactivity for H2O2. The high performance of Fe-OMC is believed to be due to the electroactive substance (iron) being incorporated in its carbon framework, as well as to the increase in the surface area of Fe-OMC.
It has been reported that the precursors containing graphitic building blocks are potentially suitable for the synthesis of graphitized carbon materials. These precursors include polyaromatic hydrocarbons, aromatic molecules, mesophase pitches, polyacrylonitrile and aromatic hydrocarbons like naphthalene, anthracene, pyrene and ferrocene. Metal particles added to carbon aerogels are believed to behave as catalysts for inducing graphitization. It is also believed that the metal catalyst may lower down the temperature of graphitization up to 1000° C. instead of conventional high temperature (2000-3000° C.) required for graphitization. The nanostructures formed may be tailored by changing carbon source, catalytic metals and even the conditions of carbonization. It has been reported that carbon aerogels with partially graphitized structure were synthesized by catalytic graphitization using Cr, Fe, Co, and Ni, and the resulting gels were mesoporous. Macroporous, low content (5%), transition metal-doped carbon aerogels reportedly have been prepared by the sol-gel method from resorcinol-formaldehyde mixtures containing the corresponding metal acetate or chloride. The transition metals specially from the iron sub group have been reported to be excellent catalysts for the graphitization of amorphous carbon due to the d-electron configuration and ionization potential of transition metals. Graphitized metal carbon aerogels in the form of nanostructured rings, onions and ribbons have been reported earlier when different transition metals were treated under various conditions. It has been reported that pyrolysis of metallocenes (e.g., ferrocene, cobaltocene and nickelocene) in the presence or absence of other hydrocarbons gives carbon nanotubes without any external metal catalyst. It has also been reported that pyrolysis gave rise to metal particles such as cobalt and iron covered by graphite sheets or carbon coated metal nano particles.
The metal particles used as catalysts for graphitization of carbon aerogels, in case catalytic particles are transition metals, have also been reported to exhibit magnetic properties; hence the aerogels may have graphitic as well as magnetic properties making them more interesting with different practical applications. Magnetic nanoparticles have attracted significant academic and technological attention because of their unique physical properties and potential applications in magnetic recordings, environmental protection, biomedicine, and magnetic resonance imaging as contrast agents, field-oriented drug delivery systems, biotoxin scavengers, as well as the magnetic fluid hyperthermia. Recently, ferromagnetic transition metal nanoparticles have boosted intensive research work, apparently because of their excellent magnetic properties (the saturation magnetization of Fe nanoparticles is twice that of magnetite, which is a very popular magnetic material). So far, numerous techniques have been developed to synthesize carbon-encapsulated iron nanoparticles (CENPs), including the arc-discharge, detonation, magnetron and ion-beam co-sputtering, RF plasma torch, mechanical milling, catalytic pyrolysis, co-pyrolysis, spray pyrolysis and chemical vapor condensation. Nevertheless, most of the aforementioned techniques require relatively drastic conditions that typically lead to operational complexity and expensive propositions.
The magnetic nanoparticles made of pure metallic phases, despite better magnetic performance, may undergo spontaneous unwanted and uncontrollable reactions: (i) surface oxidation, (ii) agglomeration, and (iii) corrosion. The specific properties of magnetic nanoparticles (that are made of pure metallic phases) may be preserved by encapsulating them in thin protective coatings. Several encapsulation agents have reportedly been proposed to enhance their stability, e.g., silica, polymers, boron nitride, gold and carbon. Polymer-coated nanoparticles are known to have limited stability at elevated temperatures and may become permeable. Silica shells frequently possess a porous structure and are easily etched in alkaline solutions. Boron-nitride (BN) and carbon coatings are free of these drawbacks. These coating agents are resistant to acids, bases, greases, oils and remain stable at high temperature (up to 650 K under a pure oxygen atmosphere). Carbon coatings, unlike their BN counterparts, are expected to be readily susceptible to chemical functionalization. Moreover, the carbon coatings on CENPs produced by these methods often do not clearly show a graphitic structure that is expected to protect effectively the metal cores.